read-cache: look for shared index files next to the index, too

read-cache: look for shared index files next to the index, too

When reading a split index git always looks for its referenced shared
base index in the gitdir of the current repository, even when reading
an alternate index specified via GIT_INDEX_FILE, and even when that
alternate index file is the "main" '.git/index' file of an other
repository.  However, if that split index and its referenced shared
index files were written by a git command running entirely in that
other repository, then, naturally, the shared index file is written to
that other repository's gitdir.  Consequently, a git command
attempting to read that shared index file while running in a different
repository won't be able find it and will error out.

I'm not sure in what use case it is necessary to read the index of one
repository by a git command running in a different repository, but it
is certainly possible to do so, and in fact the test 'bare repository:
check that --cached honors index' in 't0003-attributes.sh' does
exactly that.  If GIT_TEST_SPLIT_INDEX=1 were to split the index in
just the right moment [1], then this test would indeed fail, because
the referenced shared index file could not be found.

Let's look for the referenced shared index file not only in the gitdir
of the current directory, but, if the shared index is not there, right
next to the split index as well.

[1] We haven't seen this issue trigger a failure in t0003 yet,
    because:

      - While GIT_TEST_SPLIT_INDEX=1 is supposed to trigger index
        splitting randomly, the first index write has always been
        deterministic and it has never split the index.

      - That alternate index file in the other repository is written
        only once in the entire test script, so it's never split.

    However, the next patch will fix GIT_TEST_SPLIT_INDEX, and while
    doing so it will slightly change its behavior to always split the
    index already on the first index write, and t0003 would always
    fail without this patch.

Signed-off-by: SZEDER Gábor <szeder.dev@gmail.com>
Acked-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>